1. Field of the Invention
This invention relates to a method and apparatus for generating pick information in a graphics system and more particularly to a method and apparatus for generating visible pick information in such a system.
2. Description of the Related Art
Interactive graphics systems have two general functions, an output capability for rendering pictures and input facilities for providing interactivity. An essential part of such interactivity in many graphics system is the pick procedure, in which a user selects an object appearing on the display, such as an icon or a text display, to provide input to an application. Typically, the user positions a cursor over a portion of the display image, using a mouse or other pointing device, and presses an actuator associated with the device to make the desired selection. Picking is described in such references as Callahan et al. U.S. Pat. No. 4,847,605, entitled "Picking in a Graphics System"; Hempel et al. U.S. Pat. No. 4,870,599, entitled "Traversal Method for a Graphics Display System"; Ebbers et al, application Ser. No. 07/425,781, filed Oct. 25, 1989, entitled "Picking Function for a Pipeline Graphics System Using Hierarchical Graphics Structures", the EPO counterpart of which was published May 2, 1991, as Publication No. 425 173; and Callahan et al. European Patent Publication No. 0 318 154, entitled "A Method of Operating a Computer Graphics System", all assigned to the assignee herein, the specifications of which are incorporated herein by reference.
With the advent of real-time hidden-line/hidden-surface removal, traditional picking mechanisms are now insufficient. This is because such picking mechanisms typically use a two-dimensional (2D) position test, in which a primitive is picked if the object that it represents passes through a predetermined pick aperture, even if the object is hidden in the relevant area by other objects. Such a simple geometric test can lead to unintended results, such as where a user intending to pick an aircraft fuselage on a screen also picks a passenger seat that is entirely hidden from view. It is therefore desirable to provide additional picking selectivity such that only visible entities may be picked.